In 1976, Richard Avedon went to Washington to photograph Henry Kissinger. As Avedon was leading him to his mark, Kissinger said, “Be kind to me.” Artists have been making portraits of the mighty for centuries—from Velázquez’s Philip IV to Lucian Freud’s Elizabeth II—and the act of portrait-making can leave the royal or the tyrant, the President or the diplomat with a sudden feeling of disequilibri